1. Field of the Invention
The present invention relates to a paste composition for a solar cell, and to a manufacturing method therefor. Specifically, it relates to an aluminum-containing paste composition for forming an aluminum electrode (as a back electrode) on the back side of the light-receiving surface of crystal silicon solar cell, and to a manufacturing method therefor.
The priority claim for the present application is based on Japanese Patent Application No. 2009-297944 submitted on Dec. 28, 2009, and the entire contents of that application are incorporated by reference in this Description.
2. Description of the Related Art
A typical example of solar cell for converting the light energy of the sun into electricity is the so-called crystal silicon solar cell, a solar cell that uses crystalline (single-crystal or multi-crystal) silicon for the semiconductor substrate. An example of such a crystal silicon solar cell is the monofacial type of solar cell 110 shown in FIG. 2.
This solar cell 110 has n-Si layer 116 formed by pn junction formation on the light-receiving surface of the p-Si layer (p-type crystal silicon) 118 of silicon substrate (Si wafer) 111, antireflective film 114 of titanium oxide or silicon nitride formed by CVD or the like on the surface of layer 116, and front electrode (light-receiving surface electrode) 112 made of Ag formed typically by screen printing and firing a silver paste. Meanwhile, the back side of p-Si layer 118 is provided with back external connection electrode 122 made of Ag formed like front electrode 112 by screen printing and firing a silver paste, and aluminum electrode 120 providing a back surface field (BSF) effect.
This aluminum electrode 120 is formed over roughly the entire back surface by printing and firing an aluminum paste consisting mainly of aluminum powder. An Al—Si alloy layer (not shown) is formed during firing, and the aluminum diffuses into p-Si layer 118, forming p+ layer 124. Formation of this p+ layer 124, or in other words of a BSF layer, serves to prevent the photogenerated carrier from recombining near the back electrode, thereby improving the short-circuit current and open voltage (Voc) for example.
However, aluminum electrode 120 must be formed with a certain thickness (such as 30 to 60 μm) in order for this BSF effect to operate effectively. At the same time, silicon substrate (Si wafer) 111 and in fact the solar cell element itself are being required to be even thinner than before so that solar cell manufacturing costs can be reduced and solar cell modules made more compact.
Because of the difference between the thermal expansion coefficient of substrate 111 itself and the thermal expansion coefficient of aluminum electrode 120, however, making the substrate (Si wafer) thinner contributes to problems of warpage, curling and other deformation of the silicon substrate (wafer) 111 itself during firing to form aluminum electrode 120. Various measures have been taken in the past to prevent such warpage and other deformation.
For example, Japanese Patent No. 3910072 proposes an aluminum-containing paste composition for forming an impurity layer or electrode layer on a p-type silicon semiconductor substrate, which is a paste composition containing silicon oxide or aluminum oxide. Japanese Patent Application Laid-open No. 2008-112808 proposes a solar cell element provided with a sintered electrode consisting primarily of aluminum on a p-type silicon substrate, wherein the sintered electrode contains particles of a zinc oxide aggregate. Japanese Translation of PCT Application No. 2008-543080 (corresponding to WO 2006/132766), which does not pertain directly to the present application, proposes a paste composition for forming a solar cell contact (herein called a front electrode or back electrode), which is a paste composition containing a lead- and cadmium-free glass component.